Duplicate Annihilator works with images in your iPhoto library. It can search for duplicates in a number of ways, including MD5 checksum (the default setting) as well as (among other things) creation date, width, height, and file size. When if finds duplicates it can label them with a comment as well as move any duplicates it finds to iPhoto’s trash. To permanently delete the photos you then must Control (right) click on the Trash and choose Empty Trash.

By default, Duplicate Image Detector won’t search an iPhoto or Aperture library. Although there is an option to remove this restriction you must rebuild these libraries after you’ve removed duplicates from them. (You do that by holding down the Command and Option keys when launching iPhoto or Aperture.) Duplicate Image Detector differs from Duplicate Annihilator in that it can search within folders for duplicate images. It can also compare images in two separate folders and identify duplicates between the two folders. It makes its judgments based on images’ height and width. Once it’s identified duplicates you have the option to trash, move, alias, or view them.